Microstructures are to be found in various applications. One application area is micro-fluidics which are used in particular for chip-laboratories and so called bio-discs. For producing micro-channels, one uses, for example, a micro-structured substrate which comprises cavities on its surface. In order to finally close the cavities or channels, the surface of the substrate is covered by attaching another substrate thereto which not necessarily has to be structured.
For producing microstructures, various methods are known. One possibility of the production is provided by moulding techniques where moulding material is structured by means of a mould which comprises a negative shape of the desired structure. A liquid material is supplied into or onto a mould, hardened and, thereafter, removed again. Difficulties are encountered in producing structures of high surface quality since gas inclusions are very disturbing in view of the size of the microstructures. Gas inclusions originate mainly in the cavities of the structures as well as in the moulding material itself.
The negative shape related to the desired structure is, for example, produced by a further method of micro-technical structuring, wherein a wafer is coated with photo resist, the photo resist is exposed, developed and, finally, the wafer is structured by etching. A particularly suitable substrate for this method is silicon. With a negative mould produced in this way, further micro structures can be moulded.
It is known to apply a moulding mass unto a micro structured mould by means of a spatula. The material is hardened and, thereafter, removed from the mould. This method has the deficiency that the applied pressure is comparatively small, and that gas inclusions can be forced out only with difficulties. This method is not very complex and it can be also be considered for comparatively small number of pieces. In order to further handle the problem of the gas inclusions, a subsequent degassing in vacuum is provided. In spite of this method step, the surface quality as well as the shaping of the structures is prone to be improvement. Furthermore, the injection moulding method is known in which the moulding material is essentially fed through a screw into a cavity which comprises a micro-structured surface. Because of the pressure of the screw, the melted mass having a low viscosity is pressed into the cavities. In this method, air inclusions in the microscopic arrange are regularly encountered. This has a detrimental effect on the structures to be formed. Therefore, a complex degassing is necessary. In spite of this, the quality of the surface is anyway impaired by the gas inclusions.
A further method for producing micro structures is the so called hot stamping. In hot stamping, a polymer blank is inserted into a stamp mould. The mould is heated, and, upon reaching an appropriate temperature, the micro structure is stamped in by means of a stamp. In case of a high aspect ratio, it is indispensable also with this method to carry out the method in vacuum. The air inclusions in the moulding material are left back also in this method.
Furthermore, it is basically known from DE 34 44 996 A1, to degas a polymer by means of centrifugation. In this document, the production of a plastic part in a moulding tray is disclosed which consists out of a centrifuge tray. The mould is inserted in the tray bottom. The polymer plastics material is fed into the tray where after it is centrifuged.
This procedure has the deficiency that the emerging layer is not homogenous since the centrifugal force is only applied in the middle of the container bottom exclusively perpendicular to the bottom of the tray. Therefore, the material is shifted to the edge area which results not in a homogenous layer thickness across the all surface of the mould. Therefore, this method is not suitable since, on the one hand, moulded bodies free of bubbles can be produced and, on the hand, the tolerances are too large for producing micro structures.
Furthermore, it is known from this document, to position a plate onto the polymer in order to enlarge the forces on the polymer. This has the deficiency that the plate is arranged opposite to the direction of expulsion, and in that the gas inclusions expelled from the material are hindered in finally exiting from the polymer in the end.
Furthermore, a method is known from EP 1 512 993 A1 which describes the production of micro structures. According to this method, a polymer is applied to a rotating disc and covered floating covered with a wafer provided with a microstructure. This method ensures the production of a homogenous layer thickness. Since the centrifugal force acts in the direction to the edge areas, in order to distribute the polymer, gases are not transported to the outside according to the principle of the material separation. This method has the disadvantage that bubble free mould bodies cannot be produced.